1. Field of the Invention
This invention relates to circuits for interfacing integrated circuit outputs to circuitry external to the integrated circuit, and, more particularly, to circuits for interfacing the outputs of charge-coupled devices to external operational amplifiers.
2. Description of the Prior Art
At the output of a CMOS, PMOS, or NMOS integrated circuit chip, it is common to use an electrometer MOSFET in an emitter-follower mode as a current input to an off-chip inverting operational amplifier (op-amp). Such a configuration is particularly common in integrated circuit chips having a linear or area array of photodiode detectors, having a charge-coupled device (CCD) detector array, or having a CCD for data processing.
However, such a circuit connection having a length on the order of 1/2" results in a rather large stray capacitance-to-ground at the inverting input to the op-amp. This stray capacitance arises from the bonding pads and the wire bonds between the chips, and introduces a zero (i.e., a factor in the numerator) in the transfer function for the op-amp input voltage noise. This causes the high-frequency portion of this noise to be boosted to the point where this op-amp noise is the limiting factor in the circuit performance.
In addition, the output signal amplitude is a direct function of the electrometer transconductance. Since the electrometer transconductance is sensitive to temperature, the output signal amplitude will vary accordingly. This makes the matching of output signal from channel-to-channel very difficult since temperatures are likely to vary from channel-to-channel.
Accordingly, it is desirable to provide a chip/op-amp interface circuit with a reduced stray capacitance and having an output signal amplitude relatively insensitive to temperature variations.